When analog signals, such as current values and voltage values, output from various sensors are input into a programmable controller (PLC), an A/D conversion unit, which is an analog input unit for converting input analog data values to digital values, is used. The CPU unit of the PLC then performs various controls in accordance with the input values that have been converted to digital values.
Further, as well as digital values that are the results of A/D conversion performed by the analog input unit being used as input information for control processes performed by the CPU unit, they are also used in some cases for data collection for the purpose of, for example, traceability of quality information, i.e., logging.
With regard to the technologies related to logging performed by analog input units, an analog input unit is disclosed, for example, in Patent Literature 1 that is configured to reserve a log storage area having a ring buffer configuration in a shared memory and to store, in the shared memory, a head pointer indicating the boundary address between the latest log data and the oldest log data among the log data stored in the log storage area.
Moreover, an analog conversion device is disclosed, for example, in Patent Literature 2 that is configured such that the given-point-number logging detection unit monitors the number of pieces of logging data stored in the first storage unit and the interrupt generation unit performs an interrupt generation request with respect to the CPU unit when the number of pieces of logging data reaches a predetermined number.
With both the technologies described in Patent Literature 1 and Patent Literature 2, the CPU unit, which manages and controls the analog input unit, reads and stores a sequence of digital values collected by the analog input unit. However, there is a problem in that when the CPU unit manages and controls a plurality of analog input units and executes multi-channel logging, the data collection process is delayed in some cases and this has an effect on the control cycle of the control that should actually be performed.
Moreover, there is a technology disclosed for managing and controlling one analog input unit by a plurality of CPU units. For example, a CPU system is disclosed in Patent Literature 3 that includes, as CPU units, a first electronic control device, which mainly engages in a process related to data input/output, and a second electronic control device, which mainly engages in execution of various calculations using data.